1. Field of the Invention
This invention relates to marine bottom-supported structures useful as working platforms in cold offshore regions and more particularly to a method for constructing a grounded ice platform useful in offshore drilling and production operations.
2. Description of the Prior Art
In the continuing search for new petroleum and natural gas reservoirs, considerable interest has recently been focused on exploration and production activities in the arctic and antarctic regions of the world. In these regions, subterranean formations having a potential for petroleum and/or natural gas accumulations are sometimes found underlying ice-covered offshore locations which have relatively shallow waters. Conventional floating and bottom-supported platforms are not well suited for use in these waters.
Various types of bottom-supported platforms have been proposed for use in these regions, including platforms built with steel, concrete and/or fill material dredged from the marine bottom. While these platforms could be designed to be stable and suitable for multiseason use, such platforms are very expensive. In addition to the substantial cost of the materials employed to build such platforms, the cost of transporting these materials to the offshore location and the very high cost of construction at these locations often effectively precludes the use of such platforms. In addition, environmental considerations may necessitate the dismantling of the working platform and restoration of the offshore location to its natural state after operations on the platform are terminated. The cost of dismantling these platforms and restoration of the offshore location may exceed the cost of building the platforms. All of the aforementioned factors weigh heavily against the use of such a platform especially where the platform will only be used for a relatively short time, such as five years or less.
U.S. Pat. Nos. 4,048,808 to Duthweiler, 3,863,456 to Durning and 3,849,993 to Robinson and Durning disclose methods for building grounded ice islands in cold offshore regions. While grounded ice islands are considered suitable as a temporary, single-season working platform for exploratory drilling, the ice islands normally melt and/or break up during the warmer summer months each year. While the cost of constructing a grounded ice island is substantially less than a comparable steel and/or landfill platform, the short useful life of the known ice islands precludes the multiseason use thereof.
Various combinations of ice and steel or other construction materials have been proposed for use in constructing working platforms, such as disclosed in U.S. Pat. Nos. 4,055,052 to Metge, 3,750,412 to Fitch et al. and 3,738,114 to Bishop. However, the methods of construction disclosed therein are not as well suited to operations in cold offshore regions as the method of this invention. Some of these known construction methods require the use of heavy machinery, such as pile drivers, ditching machines, and/or dredging machines, at the offshore location but fail to provide a suitably stable platform upon which such machinery may be supported. Another problem with these known methods arises where ice and a structural steel member, such as the pilings of Fitch et al. and the outer structural wall of Bishop, are expected to freely slide one past the other for a prolonged period during the construction operations. This extended stress tends to weaken the steel member. Yet another problem with these methods is the fact that any ice movement which occurs at the site during the construction operation but before completion of the platform could result in major damage to the partially constructed platform and any other equipment located at the site. Additionally the construction methods disclosed by Bishop and Metge render it difficult to quickly dismantle the ice platform after operations thereon have been completed in order to restore the offshore location to its natural state. Thus a need exists for an improved method for constructing a low cost, multiseason working platform at an offshore location.
Accordingly, it is a primary object of this invention to provide an improved method for constructing a multiseason working platform in a cold offshore region.
Another object of this invention is to provide a method for constructing a multiseason working platform in which a stable base is established upon which any heavy machinery required in the construction method may be supported.
Yet another object of this invention is to provide a construction method in which the materials of construction are not submitted to continuous stress caused by a prolonged sliding movement imposed between dissimilar materials of construction.
A further object of this invention is to provide a construction method during which the integrity of the partially completed ice platform is relatively unaffected by local ice movements during the construction period.
Still another object of this invention is to provide a method for constructing a multiseason ice platform which may be dismantled relatively easily and quickly upon completion of the operations thereon.
Still further object, advantages and features of the invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.